Changhsingian (Late Permian) conodonts from Son La, northwest Vietnam and their stratigraphic and tectonic implications

摘要

Late Permian conodonts are for the first time reported from Vietnam. Pa, Sa, Sb, Sc and M elements of the Changhsingian conodont species Hindeodusjulfensis (Sweet) are reported from a 40 cm thick limestone in the middle part of the Yenduyet Formation near Son La, NW Vietnam. The occurrence of H. julfensis indicates a Changhsingian age that is consistent with an interpreted early Changhsingian age for a brachiopod fauna slightly higher in the sampled section. The Son La section is located in the Song Da Rift Zone and overlies basaltic volcanics considered equivalent to the Emeishan large igneous province basalts that are plume related. The Permian-Triassic boundary in Vietnam is yet to be precisely located biostratigraphi-cally but proxy chemostratigraphic data indicate its likely position in sections at Nhi Tao and Lung Cam, N. Vietnam and correlation with the Global Stratotype Section and Point at Meishan, South China. The recovered conodonts have a Conodont Colour Alteration Index of 5 and have been heated to c. 600 ℃ but they do not show any evidence of textural alteration due to regional metamorphism such as micro-folding or stretching that would indicate any direct effects of the compressional Indosinian Orogeny.